The natural endemic maintenance cycle of La Crosse (LAC) arbovirus (endemic in deciduous forests of midwestern and eastern United States) is being studied in field and laboratory for knowledge of viral, vector vertebrate and environmental factors of use for eventual control of this virus which each year is a main cause of arboviral encephalitis in children. Studies will be continued on venereal transmission of LAC virus to female Aedes triseriatus from males which have been found to be transovarially infected in nature, recently observed in our laboratory. Salivary and transovarial transmission rates obtained with LAC virus in venereally infected female Aaedes triseriatus will be studied in colonized mosquitoes and in fresh collections from endemic and non-endemic areas. Infection and dissemination of virus in dissected mosquitoes will be determined by fluorescent antibody and virus isolation techniques. Field work will focus on collection of specimens and data related to relative importance of transovarial and venereal transmission, and horizontal transfer through vector viremic engorgements on chipmunks and other vertebrates in the natural cycle of LAC virus. Epidemiologic studies of selected cases of California encephalitis (caused by LAC virus) will be conducted to learn more about the distribution and circumstances of these infections.